1. Field of the Disclosure
The disclosure relates generally to the field of radio communication and, in particular, to time delay adjustment control to match different signal paths to a common delay to aid further signal processing in applications such as coherent combining, wideband beam-forming, and wideband signal cancellation including the reduction of interference signals coupled from a transmission antenna into a local receive antenna in the presence of a local multipath.
2. Description of the Related Art
The prior art has demonstrated that an autocorrelation result between the transmitted reference signal and the continuously varied time-delayed version is indirectly proportional to the time-bandwidth product of the two multiplied signals. The time-bandwidth being defined herein as a product of the bandwidth and a time mismatch between the two signals. Such prior art systems were limited to the adjustment interval due to the system bandwidth, which limits the range of scenarios in which it is operational.
Desirably, for a large bandwidth signal, the autocorrelation result becomes highly sensitive to the measured time mismatch, which is trying to be reduced or minimized and, once locked, maintains tracking with minimal drift before having tracking restored. But, it's very sensitivity limits its range of operation such that it can hang up far from the point of matched delay and never find the proper solution due to non-monotonic function beyond the time interval of ±1/BW.
As signal bandwidths become wider, the channel matching in signal processing paths requiring joint processing becomes more critical and difficult to manufacture and maintain reliably, at low cost. This is true in beamforming and interference cancellation using multiple receive antenna elements.
In the beamforming system, the propagation of the desired signal across a large array can cause significant phase dispersion across the bandwidth of the signal. A beamsteering system is limited in its processing bandwidth and beamsteering range by the placement of elements yielding decorrelation of the desired signal across the array.
A typical beamsteering system utilizes a correlation-based adaptive controller using feedback derived after the summation process. The system takes a sample of received signal and adjusts the magnitude and phase such that the result is equal in phase with the pilot signal at the input of the receiver. The vector sum of the two signals will add, providing an enhanced signal of interest. In practice, however, the two signals have not only a phase and amplitude difference, but also a time delay yielding phase dispersion across the band, due to unwanted distortion in the propagation path length and multipath. Beamsteering performance is a function of amplitude and phase match between each sample of the desired signal received in each element. Beamsteering is more tolerant of mismatches so further explanation will focus on cancellation.
In the interference cancellation system, unwanted (i.e., interfering) signals manifest themselves in several ways. Interference can cause a reduction in the sensitivity of a receiver (receiver desensitization), masking of a desired signal, tracking of an undesired interfering signal and loss of the desired signal, and processing of the unwanted interfering signal instead of the desired signal. Each of these manifestations of interference limits the communication capabilities of the radio system afflicted by this problem. The effects of interference can be some combination of the absence of usable output from a receiver, false signals from a receiver, and malfunction of a device which is operated by the receiver. During emergency situations, the loss and corruption of the desired signal can be critical. One of the greatest limitations of the nulling of this interference is the decorrelation of the interfering signal from a sample used for subtraction.
A typical Interference cancellation system utilizes a correlation-based adaptive controller using feedback derived after the cancellation process. The system takes a sample of an interference signal and adjusts the magnitude and phase such that the result is equal in amplitude and phase with the interference signal at the input of the receiver. The vector difference of the two signals will cancel, leaving only the signal of interest. In practice, however, the two signals are not identical, due to unwanted distortion in the reference path, as well as differences in signal path lengths and non-ideal components in the Tx/Rx signal paths. Cancellation performance is a function of amplitude and phase match between the interference signal and the sampled signal. To suppress a wideband interference signal, the performance of a cancellation system is directly proportional to the path length match between the cancellation signal and the interference signal. For a modulated carrier, phase match is determined by the signal bandwidth and the path length difference in time (often measured in nanoseconds) between the transmit antenna to the receive antenna signal path and the coupled transmit signal path to the receive antenna.
In order to provide good signal suppression, the cancellation signal path length should be adjusted to time match the interference signal path length. Contemporary techniques for this adjustment involve manual measurements to determine the path length difference between the interference cancellation signal and the received interference signal. One prior art solution utilizes a manually controlled trombone to extend a tap to a proper delay. This solution is deficient to the extent that it only provides an approximate solution for a stationary environment and requires manual intervention. Conventional manual methods do not lend themselves to situations where the time match may change over the course of time as a result of a changing environment. Thus, a conventional interference cancellation system may suffer a loss of suppression performance due to system dynamics.
One prior art solution proposes the use of an adaptive time delay system which provides time-matched delays in auxiliary paths by providing additional taps in the auxiliary paths to cover a range of possible time delays. However, the taps only cover a discrete set of time-delays resulting in less than optimal results.
A further prior art solution proposes the use of a digital search method that requires an interrupt of the system operation for adjustment. This solution is deficient by requiring both system interruption and manual intervention.
A more recent implementation, described in U.S. Pat. No. 8,094,764 B2, incorporated herein by reference in its entirety, describes a method for fine tuning the channel match in a system for interference cancellation. However, it is limited to a time delay adjustment range of ±1/BW of the signal of interest, which limits the variability of scenarios in which it can work without manual intervention.
A need therefore exists for a system and method for continuously adjusting a matching time delay where the time match may change over the course of time as a result of a changing environment over an adjustment range wider than ±1/BW, as described in U.S. Pat. No. 8,094,764 B2.